Igneous Rocks
Introduction
 Igneous Rocks – rocks that appear to have been molten in the past
 Molten rock names (based on location)
 Magma: molten rock found anywhere beneath the earth’s surface
 Lava: molten rock that flows out onto the earth’s surface
 Molten rock types (based on where they are formed)
 Extrusive rocks: form when lava solidifies above the earth’s surface
 Intrusive rocks: form when magma solidifies after squeezing into other rocks,
intruding into them (may eventually appear at earth’s surface due to erosion)
Texture and Composition
 Texture - Sizes of mineral crystals are the main difference between intrusive
and extrusive rocks.
 Coarse: larger crystals
 Fine: small crystals
 Glassy: no crystals
 Factors determining crystal size:
 Number of foreign particles present
around which crystal accretion can
begin (nucleation sites)
 Concentration of various elements
in melted rock
 Temperature
 Pressure
 Rate of crystallization
Texture and Composition
 Intrusive igneous rocks tend to have textures that are coarse to fine
because conditions deep underground encourage large crystal formation.
 Extrusive rocks tend to have fine to glassy textures.
 Composition:
 Depends on the mineral content of the molten rock from which they solidify
 “Felsic” contain much silica
 Examples – light colored rocks like feldspars and quartz
 “Mafic” rich in iron and magnesium
 Examples – dark colored rocks like augite, olivine, and biotite mica
 Igneous rocks may fall anywhere between these extremes
Igneous Intrusions
 Intrusions = intrusive structures underground
 Largest intrusion is called a batholith (hundreds of square kilometers
horizontally and several thousands of meters thick!)
 Many mountain ranges are built on top.
 Stock – a batholith that is less than 100 square kilometers
 Pluton – an intrusive rock formation deep in the earth
Igneous Intrusions
 Laccolith – the solidified formation made when magma intrudes in such a
way that it pushes overlying strata into a dome
 Multiple laccoliths make low hills.
 Sills – sheets of magma that has forced its way between layers of existing
rock
 Vary from centimeters to meters in thickness and can extend
for kilometers
 Dikes – hardened magma that intruded into
cracks that cut through existing layers of rock
 Often associated with volcanic activity
Intrusive Igneous Rocks
 Granite – “containing grains,” most common, coarse-grained, felsic rock
 Colors: pink, white, black
 Fractures may be smooth or rough (depending on composition)
 Famous building material
 Other intrusive igneous rocks include gabbro and diorite (coarse texture)
 Porphyry = rock containing large crystals embedded in smaller crystals
(probably formed in two different stages of cooling)
Extrusive Structures
 Lava flow – magma that extrudes onto earth’s surface
 Lava plateau – thick rock formation from lava flooding hundreds of square
kilometers of land
 When a volcano becomes dormant, the lava usually solidifies within it.
 The vertical column-like formation that remains after erosion removes
overlying volcanic debris is known as a volcanic neck.
 Example: Shiprock in New Mexico
Extrusive Igneous Rocks
 “Lava Rocks” solidify from molten lava.
 Basalt = dark, mafic extrusive rock with fine crystal texture
 Abundant throughout the world
 Colors: dark greenish gray to black
 Rapidly solidified near earth’s surface or in the neck of a volcano
 Patterns of long fractures from stress of cooling… look like multisided
columns
Extrusive Igneous Rocks
 Felsite is lighter in color and contains silicates.
 Scoria is frothy with dissolved gases (cools so fast that gas bubbles are
trapped) and looks like dark cinders. Often mistaken for meteorites.
 Pumice is similar to scoria, but lighter in color. It can float in water because
it contains so many trapped bubbles!
 Obsidian is of the granite family, but it forms when degasified lava cools
rapidly at near-atmospheric pressures.
 “Natural glass”
 No crystals
 Colors: black, brown, and red most commonly
 Conchoidal fracture